A propulsive assembly of an aircraft such as a turbojet engine comprises several systems that need to be cooled or held at an optimal operating temperature such as the turbomachine and the electrical apparatuses like the electric generator of the aircraft.
It is also necessary to limit the temperature of the lubricating oil for the moving parts of the turbomachine and to remove the heated given off by the bearings and frictional parts.
The systems traditionally cooled or temperature-controlled by a cooling circuit in the engine are the electric generators coupled to the engine (turbomachine) and the moving parts of the engine.
Two cooling principles are also known that are generally used for propulsion systems.
The first consists of cooling by oil/air heat exchange, with a heat exchanger arranged in a shunt circuit that draws air from the secondary cold flow of the engine.
This first principle is disadvantageous for the efficacy of the propulsion assembly because it takes air from the engine and/or it introduces a loss of supplementary aerodynamic pressure. To limit this drawback, it is now acceptable to integrate a valve in the exchanger to regulate the flow of air taken from the engine. Nevertheless, these regulating valves detract from the global reliability of the cooling system and are the origin of numerous service problems (appearance of cracks in the valves and the pipes because of aerodynamic vibrational stresses, occurrence of valve control system failures, etc.).
This first principle is also disadvantageous for the acoustic handling of the internal surface of the secondary flow. Actually, the larger the size of the exchanger to be integrated, the more air will enter it (and leave it if the flow of tapped air is discharged into the secondary flow), since the intake and discharge of air have no acoustic treatment, and therefore it is desirable to keep their amounts low to control engine noise.
The presence of the exchanger is unfavorable because it opposes reducing the dimensions of the air inlet and outlet.
The second principle is to use the fuel that feeds the engine as coolant, and in this case, one or more fuel/oil exchangers are used, traditionally of the plate or tube types of exchangers, which are integrated in the propulsion assembly.
These exchangers provide for dissipation of the heat energy in the fuel used by the engine.
Still, the fuel must not be heated beyond a certain temperature (˜150° C.) so as not to involve any risk of coking. For this reason, some propulsion assemblies draw an amount of fuel from the reservoirs of the aircraft that is much greater than the actual need of the engine for combustion, and return the unused heated fuel to the reservoirs.
As for the air/oil exchanger devices, the fuel/oil exchangers cannot generally be arranged close to the devices to be cooled, and here again the oil circuits have to be lengthened between the devices to be cooled and the exchangers.
In conclusion, regardless of the principle used, the devices to be cooled, or heat sources, are cooled and regulated through the expediency of their own lubricating systems, which implies lengthening these circuits to the exchangers remote from these sources of heat.
The oil circuits also have a dual function of lubricating and cooling in the prior art.
Because of this dual function, the oil circuits of the various components to be cooled must imperatively be segregated to limit the risks of combined failures (contamination of one oil circuit involving the contamination of another circuit, leakage in one oil circuit involving the total loss of circuits, etc.), which further increases the lengths and number of pipes for oil circulation.
To the extent that each cooling circuit is dedicated to a specific apparatus (engine or electric generator), each circuit also has to possess at least one cooling device of a size for the most compelling cooling (example: maximum electric consumption under “hot weather” conditions with the airplane on the ground). Since not every cooling circuit is necessarily subject to the most compelling case in the same phases of flight, the cooling devices are almost never used 100% at the same time. There is then excess cooling capacity installed on the propulsion assembly, which is unfavorable for the performance of the propulsion assembly to the extent that the segregation rule has to be complied with, and increases its weight and its volume.
Furthermore, the fact that in the known devices the cooling circuit constitutes in parallel the lubricating circuit for the heat sources imposes several constraints on the integration of said circuit. First of all, to the extent that the oil circuit has to be linked to the heat source at the different exchangers that are not necessarily close to one another, the volume, length, and complexity of the oil circuit imposes pressure losses in the circuit and a substantial volume of oil. In other respects, the oil circulation all around the engine increases the risk of leaks, contamination, and fires in the propulsion assembly, which means a certain vulnerability of the cooling system and of the associated propulsion assembly.
Finally, the lubricating oil is not the most appropriate liquid for the transport of heat energy because of its significant viscosity and its non-optimal heat capacity, and because the cooling circuit of each heat source is cooled and temperature-controlled through the expediency of its own lubricating circuit, the shortest possible lubricating circuit is imposed to limit the pressure losses and the risk of leaks. It is then difficult to envisage connecting the cooling system(s) of the various components of the propulsion assembly, and even more difficult to envisage connecting them to those of the airplane. Thus no synergy is possible between the cooling capacities of the airplane and of the propulsion assembly, or between the heat sources of the airplane and of the propulsion assembly, which impedes any communication between the thermal devices, but such synergies would permit more extensive optimization of the cooling systems.
FIG. 1 shows an example of a system of the prior art.
In this example, the lubricating oil circuit 11 flows in the engine nacelle to reach the coolers 12, 13 of the fuel/oil heat exchanger type, and the lubricating oil circuit 14 of the electric generator extends to the cooler 15 of the fuel/oil type located in the recirculation circuit for the engine fuel supply.
FIG. 2, also of the prior art, shows an aircraft engine 1 that has a nacelle 2 and a propulsion assembly 3 provided with air/oil exchangers 6, 8 located in the conduits 7, 9 and 4, 5 diverting a portion of the secondary flow to cool the exchangers.